Dielectric coatings play an important role in achieving desired performance of III--V or II--VI semiconductor optoelectronic devices. Dense, closely packed thin films are required to protect the surface, such as light emitting or receiving facets, of optoelectronic devices from contamination and oxidation. Antireflection coatings (AR) are required on light emitting or receiving facets to increase the quantum efficiency of optoelectronic devices. Dielectric thin films providing low midgap interface state density are required, in particular on light emitting facets, to minimize nonradiative energy-dissipating processes such as carrier recombination via interface states. Carrier recombination is known to trigger a process at laser facets called thermal runaway causing device failure especially when operated at high optical power. Inversion channel field effect devices require dielectric films providing an unpinned Fermi level and low density of interface states below midgap (p-channel device) or above midgap (n-channel device) at the dielectric/semiconductor interface. Further, hysteresis-free capacitance-voltage characteristics with excellent reproducibility of flatband voltage, small flatband voltage shift, and small frequency dispersion are required. Also, passivation of states on exposed surfaces of electronic III-V devices require low density of midgap interface states.
A variety of materials has been proposed for such layers including ZrO.sub.2, Al.sub.2 O.sub.3, SiO.sub.x, SiN.sub.x, SiN.sub.x O.sub.y, Y.sub.2 O.sub.3 stabilized ZrO.sub.2, borosilicate glass and gallium oxide. The SiO.sub.2 and SiN.sub.x layers are usually deposited by sputtering, which can cause damage to the semiconductor surface. Electron-beam deposition of coatings such as Al.sub.2 O.sub.3 or ZrO.sub.2 requires addition of oxygen to get the proper stoichiometry for a desired refractive index. This requirement makes it difficult to form the layer reproducibly.
Al.sub.2 O.sub.3, SiO.sub.x, SiN.sub.x, SiN.sub.x O.sub.y, and borosilicate glass layers are fabricated with dielectric properties, but exhibit a pinned Fermi level near midgap with a midgap state density above 10.sup.13 cm.sup.-2 eV.sup.-1 when deposited on bare III-V semiconductor layers. The midgap interface state density is in a range between 7.times.10.sup.11 cm.sup.-2 eV.sup.-1 and 10.sup.13 cm.sup.-2 eV.sup.-1 when deposited on GaAs samples previously treated by liquid or dry surface passivation techniques. The long term stability of liquid passivated semiconductor/dielectric interfaces under thermal stress has yet not been investigated. Furthermore, large hysteresis (at least a few volts), nonreproducible flatband voltage shifts (at least a few volts), large frequency dispersion of capacitance, and high interface state densities closer to valence or conduction band edge, did not yet allow fabrication of inversion channel field effect devices on III-V semiconductor devices. On the other hand, gallium oxide thin films deposited in an oxygen radio frequency plasma in a vacuum system, in conjunction with a GaAs surface previously treated by H.sub.2 and N.sub.2 plasma, gives dielectric/GaAs interfaces with midgap density of states well below 10.sup.11 cm.sup.-2 eV.sup.-1. The realization of inversion channel field effect devices has been prevented in this case by large hysteresis (.gtoreq.2 V), nonreproducible flatband voltage shift (between 2 and 10 V) and leaky gallium oxide films.
It is therefore an object of the invention to provide a proper coating for protection and optical anti-reflection providing low density of midgap interface states when deposited on bare III-V semiconductor surfaces, in particular on light emitting facets for improved device reliability. It is another object of the invention to provide a dielectric thin film in field effect devices for inversion channel applications on III-V semiconducting substrates.